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.Dd January 24, 2023
.Dt GETCONTEXT 2
.Os
.Sh NAME
.Nm getcontext ,
.Nm getcontext_extd ,
.Nm setcontext
.Nd get and set current user context
.Sh SYNOPSIS
.In ucontext.h
.Ft int
.Fo getcontext
.Fa "ucontext_t *ucp"
.Fc
.Ft int
.Fo getcontext_extd
.Fa "ucontext_t *ucp"
.Fa "uint32_t flags"
.Fc
.Ft int
.Fo setcontext
.Fa "const ucontext_t *ucp"
.Fc
.Sh DESCRIPTION
The
.Fn getcontext
function initializes the structure pointed to by
.Fa ucp
to the current user context of the calling process.
The
.Vt ucontext_t
type that
.Fa ucp
points to defines the user context and includes the contents of the calling
process' machine registers, the signal mask, and the current execution stack.
.Pp
The
.Vt ucontext_t
structure is a part of the system ABI.
However, most architectures have added additional register states such as the
extended vector and floating point registers that are not part of that.
To facilitate getting that state
.Pq such as the x86 xsave area
the
.Fn getcontext_extd
function exists.
Once called, the context will be initialized and is suitable for use in other
context operations just as though one had called
.Fn getcontext .
.Pp
When calling the
.Fn getcontext
function the
.Vt ucontext_t
is completely overwritten without regards for what is currently present.
This is different when using
.Fn getcontext_extd .
Instead, the
.Vt ucontext_t
structure is read by the kernel and it assumes that the user has
initialized it.
This allows the system to consider members of the
.Vt ucontext_T
.Po
such as the
.Fa uc_xsave
member on x86
.Pc
to point to properly sized memory.
.Pp
To allow for all extended states to be copied out,
.Fa ucp
must be allocated with
.Xr ucontext_alloc 3C .
Otherwise whether it is declared on the stack, as global data, allocated
dynamically, or part of a structure,
.Fa ucp
must be zeroed through a call to
.Xr bzero 3C
or
.Xr memset 3C
prior to calling
.Fn getcontext_extd .
Improper initialization can lead to memory safety bugs, making it critical that
this is done.
.Pp
The
.Fa flags
member must be zero and is present to allow for what is copied out to change in
the future.
This indicates that the system should attempt to copy out all extended states,
though if the
.Vt ucontext_t
was not allocated with
.Xr ucontext_alloc 3C ,
some extended states may not be.
This happens because
.Xr ucontext_alloc 3C
takes care of allocating and setting up the
.Vt ucontext_t
to indicate that memory beyond the
.Vt ucontext_t
is valid and the corresponding flags in the structure are set.
.Pp
The
.Fn setcontext
function restores the user context pointed to by
.Fa ucp .
A successful call to
.Fn setcontext
does not return; program execution resumes at the point specified by the
.Fa ucp
argument passed to
.Fn setcontext .
The
.Fa ucp
argument should be created either by a prior call to
.Fn getcontext ,
or by being passed as an argument to a signal handler.
If the
.Fa ucp
argument was created with
.Fn getcontext ,
program execution continues as if the corresponding call of
.Fn getcontext
had just returned.
If the
.Fa ucp
argument was created with
.Xr makecontext 3C ,
program execution continues with the function passed to
.Xr makecontext 3C .
When that function returns, the process continues as if after a call to
.Fn setcontext
with the
.Fa ucp
argument that was input to
.Xr makecontext 3C .
If the
.Fa ucp
argument was passed to a signal handler, program execution continues with the
program instruction following the instruction interrupted by the signal.
If the
.Fa uc_link
member of the
.Vt ucontext_t
structure pointed to by the
.Fa ucp
argument is
.Dv NULL ,
then this context is the main context, and the process
will exit when this context returns.
The effects of passing a
.Fa ucp
argument obtained from any other source are unspecified.
.Sh RETURN VALUES
On successful completion,
.Fn setcontext
does not return and
.Fn getcontext
and
.Fn getcontext_extd
returns 0.
Otherwise, -1 is returned.
.Sh ERRORS
No errors are defined for
.Fn getcontext
or
.Fn setcontext .
.Pp
The
.Fn getcontext_extd
function only sets
.Va errno
in some circumstances when it fails.
The function may fail if:
.Bl -tag -width Er
.It Er EINVAL
.Fa flags
had invalid values.
.El
.Sh USAGE
When a signal handler is executed, the current user context is saved and a new
context is created.
If the thread leaves the signal handler via
.Xr longjmp 3C ,
then it is unspecified whether the context at the time of the corresponding
.Xr setjmp 3C
call is restored and thus whether future calls to
.Fn getcontext
will provide an accurate representation of the current context, since the
context restored by
.Xr longjmp 3C
may not contain all the information that
.Fn setcontext
requires.
Signal handlers should use
.Xr siglongjmp 3C
instead.
.Pp
Portable applications should not modify or access the
.Fa uc_mcontext
member of
.Vt ucontext_t .
A portable application cannot assume that context includes any process-wide
static data, possibly including
.Va errno .
Users manipulating contexts should take care to handle these explicitly when
required.
.Sh INTERFACE STABILITY
.Sy Committed
.Sh SEE ALSO
.Xr sigaction 2 ,
.Xr sigaltstack 2 ,
.Xr sigprocmask 2 ,
.Xr bsd_signal 3C ,
.Xr makecontext 3C ,
.Xr setjmp 3C ,
.Xr sigsetjmp 3C ,
.Xr ucontext_alloc 3C ,
.Xr ucontext.h 3HEAD ,
.Xr attributes 7 ,
.Xr standards 7
